BatteryStats: Don't print empty platform idle state

Bug:28863586
Change-Id: Iac1fae2063498337c7325011603933aa4f6ebfdd
diff --git a/core/java/android/os/BatteryStats.java b/core/java/android/os/BatteryStats.java
index 4acb729..54297e5 100644
--- a/core/java/android/os/BatteryStats.java
+++ b/core/java/android/os/BatteryStats.java
@@ -4922,7 +4922,9 @@
                         pw.print(',');
                         pw.print(rec.stepDetails.statIdlTime);
                         pw.print(',');
-                        pw.print(rec.stepDetails.statPlatformIdleState);
+                        if (rec.stepDetails.statPlatformIdleState != null) {
+                            pw.print(rec.stepDetails.statPlatformIdleState);
+                        }
                         pw.println();
                     }
                 }